The present invention relates to a semiconductor device having a bipolar transistor structure which has a fast switching speed and can reduce power consumption. More specifically, the present invention relates to a semiconductor device having a bipolar transistor which contains a Zener diode at its base, can be used as a digital transistor, or makes it possible to provide a fast switching with low power consumption, while providing a large current capability, with a structure of stripe emitters, multi-emitters or multi-bases.
The principled structure of bipolar transistors heretofore used is the one shown in FIG. 13. That is, formed in the surface layer portion of an n-type semiconductor layer 11, is a p-type base region 12, within which an n-type emitter region 13 is formed. Joined to the base region 12 is a base electrode 16 through a contact region 15 consisting of p+-type region, and joined to the emitter region 13 is an emitter electrode 17. A collector electrode 18 is provided on the back of an n+-type semiconductor substrate 11a under the n-type semiconductor layer 11. Numeral 19 designates an insulation film.
A bipolar transistor Tr with this structure, when used as a switching circuit, for example, shown in FIG. 14 (a), is connected though a resistor R1 to a power voltage VCC, wherein when a signal is inputted into the base, a base current IB flows, and thus the transistor Tr is operated to cause a collector current IC to flow. When considering a backward withstand voltage of the transistor, a backward withstand voltage BVCEO between collector/emitter with the base opened should be larger than the power voltage VCC, and a backward withstand voltage BVCBO between collector/base with the base opened has been known to generally have a relationship with the BVCEO of the following equation (1), because a somewhat current flows even with the base opened, and thus a current amplification factor hFE is effective. FIG. 14(b) is a circuit diagram of an example consisting of a digital transistor described later.
VCC less than BVCEO=BVCBO/(1+hFE)1/nxe2x80x83xe2x80x83(1) 
For example, trying to use the backward withstand voltage BVCEO between collector/emitter at 24 V, a design must be made so that the backward withstand voltage BVCBO between collector/base exhibits 60 V. To make this backward withstand voltage BV between collector/base higher, it is necessary to make the impurity concentration of the semiconductor layer 11 lower with a structure shown in the above-mentioned FIG. 13, and make larger the thickness d of the n-type semiconductor layer 11 located on the lower side of the base region 12 so that a depletion layer is sufficiently widened. Making lower the impurity concentration of the n-type semiconductor layer 11 or making thicker the thickness d thereof causes the series resistance between emitter/collector to be increased. On the other hand, a voltage VCE(sat) between collector/emitter when the transistor is allowed to operate to reach a steady state becomes larger in proportion to the BVCEO, that is the BVCBO as shown in the following equation (2), so that trying to make larger the backward withstand voltage BVCBO between collector/base causes also the VCE(sat) to become larger.
VCE(sat)xe2x88x9dBVCEO=BVCBO/(1+hFE)1/nxe2x80x83xe2x80x83(2) 
On the other hand, the power consumption becomes a product of the voltage VCE(sat) between collector/emitter in a steady state by the collector current IC, so that the power consumption becomes larger by that portion if the withstand voltage BVCBO between collector/base is made larger.
In transistors requiring a large current, since the collector current relates mainly to the area and peripheral length of the emitter, so that a transistor structure can be assumed in which the area and peripheral length of the emitter are made large and which has multi-emitters, multi-bases or stripe emitters reducing the current density, but such structure not so contribute to the improvement in switching speed or the reduction in power consumption.
Further, where a voltage-drive type transistor such as a digital transistor is configured using such a bipolar transistor, for example, as shown in FIG. 14(b), the voltage-drive type transistor is formed of a circuit in which when a predetermined voltage is applied through dividing resistances R1, R2 to the base B of a bipolar transistor Q, the transistor Q is turned on, while when a predetermined voltage is not obtained, the transistor Q is not turned on. However, the bias setting by such dividing resistances provides a problem in that the speed is delayed due to the load capacity of the resistances and the like. Also, even when connecting a Zener diode externally to the base of the transistor, a capacity develops due to connecting lead and the like, thereby causing speed to become lower.
As described above, a bipolar transistor of this type has a problem in that trying to improve the backward withstand voltage causes the operating voltage (VCE(sat)) to be also raised, thereby making power consumption large.
Further, although, in the above-mentioned transistor structure, the connecting portion of the base region with the base electrode 16 is formed with the p+-type contact region 15 in order to obtain an ohmic contact by increasing impurity concentration, a higher impurity concentration in the contact region 15 causes electrons of a few number carriers to be blocked by the p/p+ junction between the base region 12 and the contact region 15, whereby at switching operation, an electron accumulation develops in the base region 12. This provides a problem in that the switching loss becomes large, thereby preventing a fast switching (in particular, making off time longer), and further increasing power consumption.
Voltage-drive type transistors such as digital transistors using conventional bipolar transistors have a problem in that bias setting is made by resistance dividing, so that the load capacity becomes large and thus the speed is lowered.
The present invention is made to solve such problem, and an object of the present invention is to provide a semiconductor device having a bipolar transistor which has a high withstand voltage and can reduce power consumption.
Another object of the present invention is to provide a semiconductor device having a transistor which can obtain the fast switching speed and the large current.
Still another object of the present invention is to provide a voltage-drive type bipolar transistor, such as a digital transistor, which has a small load capacity while setting (establishing) a disired drive voltage.
Yet another object of the present invention is to configure a circuit using a Zener diode and a transistor for protecting against overvoltage and the like, by a discrete package containing two normal elements.
A semiconductor device according to the present invention has a bipolar transistor structure including; a first conductivity type semiconductor layer taken as a collector region, a base region consisting of a second conductivity type region provided in said first conductivity type semiconductor layer, an emitter region consisting of a first conductivity type region provided in said base region, a base electrode connecting portion having a first conductivity type provided in said base region, a base electrode provided on the surface of said base electrode connecting portion, and an emitter electrode and a collector electrode provided and electrically connected to said emitter region and said collector region, respectively.
With this structure, the semiconductor region is formed which conductivity type is different from that of the base region, between the base electrode and the base region, so that as a semiconductor structure, the structure between collector/base and that between collector/emitter become substantially the same. And since a reverse bias p/n junction is formed between base electrode and base region, so that no base current flows with the base opened. Thus, the backward withstand voltage between collector/base with the base opened becomes substantially the same as the backward withstand voltage between collector/emitter with the base opened.
That is, without requiring to increase the backward withstand voltage between collector/base by a voltage based on the transistor current amplification factor, the impurity concentration of the first conductivity type semiconductor layer taken as a collector region can be adjusted so as to obtain the backward withstand voltage between collector/emitter. As a result, without requiring to increase the resistance of the first conductivity type semiconductor layer to a value larger than is required, the operating voltage is lowered, thereby reducing power consumption.
The emitter region may be formed in the form of a plurality of stripe shape regions, and the base electrode connecting portion is formed along the stripe shape regions in the base region between the plurality of stripe shape regions; or the base region may be formed so as to be exposed in a matrix form in said emitter region, and the base electrode connecting portion is formed in each of the base region exposed in a matrix form; or the emitter region may be formed so as to have a plurality of regions which are exposed in a matrix form in the base region, and the base electrode connecting portion is formed in the base region adjacent to each of the plurality of regions.
By these constructions, with a large current transistor structure, the switching speed can be increased while lowering power consumption. In this case, the emitter electrode connected to the emitter region and the base electrode connected to the base electrode connecting portion are formed in an alternately meshed comb-tooth form, whereby an increase in capacity due to connecting wiring and the like can be prevented.
The base electrode connecting portion is formed by the diffusion from a circular opening having a diameter two times or less than the diffusion depth, or from an elongated opening having a width two times or less than the diffusion depth and having circular arc ends taking the width as a diameter, whereby the base current per unit cell can be increased and thus a large current can be attained. Now, circular does not mean a complete circular, but means a shape in which a partial corner portion is not formed, and including of also a shape appearing like an ellipse. The elongated opening means including of those in which a plurality of elongated openings (portions) are overlapped, not limited to one portion.
A transistor according to the present invention comprises; a first conductivity type semiconductor layer taken as a collector region, a base region consisting of a second conductivity type region provided in the first conductivity type semiconductor layer, an emitter region consisting of a first conductivity type region provided in the base region, a base electrode connecting portion having a first conductivity type provided in the base region, a base electrode provided on the surface of the base electrode connecting portion, and an emitter electrode and a collector electrode provided and electrically connected to the emitter region and the collector region, respectively, wherein Zener diode is formed so as to be broken down at a desired voltage at the p/n junction between the base electrode connecting portion and the base rejoin, by adjusting the impurity concentration of the base electrode connecting portion, said Zener diode being incorporated in series with the base.
With this structure, the transistor does not operate until the breakdown voltage of the Zener diode is exceeded. Thus, a voltage-drive transistor such as a digital transistor which does not operate unless applying a voltage equal to or larger than a predetermined value by adjusting the breakdown voltage can be embodied at a fast drive without introducing an increased capacity due to resistance dividing and the like.
By comprising the first transistor with the Zener diode aforementioned, and a second transistor whose base is connected to the collector or emitter of the first transistor, or an MOS transistor whose gate is connected to the collector or emitter of the first transistor, wherein the first transistor and the second transistor or the MOS transistor are contained in a single package, overvoltage protective device having the Zener diode and two transistors can be formed with discrete high withstand voltage, while using an usual package containing two elements.